The tumor-suppressing mechanism of cellular senescence is a program of terminal growth arrest that is triggered by various forms of stress stimuli. The onset of oncogene-induced senescence is regulated by the tumor suppressor protein p53. It has been shown that oncogenic ras and p53 cooperate to induce cellular senescence [1]. The signaling from oncogenic ras causing abnormal MAP kinase activity to p53 is not fully understood, but a linear pathway from oncogenic ras to p53 via p19ARF-Mdm2 is supported by the available data. Consistent with the critical nature of the ARF-Mdm2-p53 pathway Nutlin-3a, a small molecule antagonist of Mdm2 has been shown to trigger senescence in several human tumor cell lines [2]. It has been demonstrated that MdmX, a homolog of Mdm2 that also represses p53 activity, can block oncogene-induced senescence in mouse embryonic fibroblasts [3] suggesting a role for MdmX in the senescence pathway. This study focuses on testing the hypothesis that the overexpression of human MdmX (HdmX) blocks oncogene-induced senescence in human diploid fibroblasts (IMR90\#8217;s). Infection of IMR90\#8217;s with oncogenic H-ras induced senescence in over 50% of the cells, as measured by \#946;-galactosidase activity. Upon co-infection with HdmX, senescence levels decreased to 20% (p < .05). Given that HdmX can associate with p53, Hdm2 and potentially p19ARF, we created site-directed mutants that prohibit HdmX association with p53 (HdmXG57A) and Hdm2 (HdmXC437G). When co-infected with H-ras neither HdmxG57A nor HdmxC437G was capable of inhibiting ras mediated senescence (p < .05). It appears from our present findings that HdmX requires the ability to associate with both Hdm2 and p53 to inhibit cellular senescence. Current studies are examining the cellular localization and protein interactions of these HdmX site-directed mutants as well as testing the impact of loss of HdmX on cellular senescence in human tumors that overexpress HdmX.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3472.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO